First off, I've got a 98 Civic EX. When I first start the car up, its fine until it warms up. After it has warmed up and I come to a stop, the idle is fine (about 600 rpms) for around 45 seconds, then my break pedal suddenly looses a LOT of pressure and sinks down a lot, and my idle drops down to 100-200 rpms. Somehow it has never died on me when it drops. I'm not sure how it still runs at such a low rpm. It doesn't happen every time I stop, but fairly frequently.

Any ideas as to what it could be and what I can do to fix it? It runs fine other than this, and like I said, its never died on me.

 

well your motors NEVER idled that low... count the RPM needles.... when the motors running it at least registures 500RPM so youd be runnen around 6-700 rpms.

and ive noticed the same thing every now and then when the brake pedal will dip down.... but its never been a problem holding the brakes... i figure its just the vacume dieing down from the idling so long... the higher your motors turning the more vacume ya get. so its not realy a problem....

 

I should take a video of it... I stop, it idles where it should (around 600rpm) then the rpm meterdrops down to RIGHT above the first white mark. The engine sounds bogged down when this happens too. Yeah, the brake thing doesn't bother me too much either, it happens in most cars. What bothers me is it happens in sync with the rpms going down.

It makes me think there's some weird pressure problem or something....

 

again man your not idling below 600rpm.... EVERY line is 500rpm... ya have - then 1 then - then 2 then - then 3.... blablabla those lines are 500rpm marks if you where ideling below 500rpm you needle would look like it does with the motor OFF....
typical idle is around 1K with no accessories running it will drop to around 600rpm.

 

the brakes are falling away?? you need a new master cylender(pardon my spelling). if there is pressure at first when you apply the brake then after your stopped and the pedel gets soft and falls to the floor that means brake fluid is passing by the rubber diaphrams in the master cylender and you HAVE to replace it or else itll just go, and youll all off a sudden have zero brakes exept the ebrake. but as for the idle? thats a toughy, could be several things. do you have antilock brakes? is your ac working too hard? there is way too much to consider when it comes to diagnosing an idle problem over the internet, have it checked it out and good luck[8D]

 

id say your wrong....... thats a waste of money to replace a master cyliner! its nothing more than low vacume at low idle speed very common you dont need any more brakes when your stoped and idling at a stop light....
your trying to turn alittle kwirk into a major repair when it wont change anything.... thats just how it is...


or dont listen to me and waste all that money to fix nothing... im just trying to help ya out....

i have felt the same thing... the brakes dont FALL AWAY and to the floor... they simply soften up the pedal alittle bit and it goes down MAYBE 1/2-1inch.
only time you would need to replace the master is if you loose brake pedal COMPLETLY. and if that happens YOULL KNOW IT.LOL

 

ok ok ok. hahah. well my mc didnt just fall out one day it was gradual. it happened over a period of abou a week or so. it go progressivly worse so all im sayin if your stopped then the pedel goes away. thats the problem. but dont listen to me i only have one star haha

 

Mybooster fell out in the coarse of a day. I drove it for a week, my mom had it for a day, got it back the following day and she's talking about how it's f---ed up. Replaced the MC with no luck, then replaced the booster and turns out that it was the booster. Dramatically improved vaccum for idle also. EDIT: I also heard a nasty hisssing sound coming from brake pedals before I chose to replace anything. I know, it's sad.